New directions in x-ray photoelectron diffraction: adsorbate and semiconductor structures, valence-level and inelastic photoelectron diffraction, and photoelectron holography

Abstract

This dissertation consists of a series of projects dealing both with the structure of metal and semiconductor surfaces and interfaces as studied with photoelectron diffraction (PD), and with several new and interesting directions of research using PD. These basic techniques involved are scanned-angle x-ray PD (XPD), scanned-energy PD (also known as angle-resolved photoemission fine structure or ARPEFS), and photoelectron holography (PH). In these techniques, modulations of the intensities of core-level photoelectrons emitted from a single crystal surface are produced by scattering and interference effects when either the angles of emission above the surface are varied (XPD and PH) or the photon energy and the detected photoelectron energy are scanned synchronously (ARPEFS). Simple single-scattering (SS) and more complex multiple-scattering (MS) calculations can be used to determine surface structures in both XPD and ARPEFS. In PH, full sets of XPD data over a large solid angle are used to create a hologram which can be mathematically inverted by Fourier transformation (FT) to directly produce images of atomic positions near surfaces. In this dissertation a series of different systems have been studied using different PD techniques. The controversial (√3x√3)R30° structure of Ag on Si(11l) has been studied with a complete set of azimuthal- and polar- XPD scans of Ag 3d5/2 and Si 2p intensities, and an extensive set of ARPEFS Ag 3d data. These data have been analyzed with both SS and MS calculations to better determine the atomic positions in this system. Through the use of XPD, the HgCdTe(111) surface also was studied. Due to the lack of a center of symmetry in the zinc-b1ende lattice of HgCdTe, these (111) surfaces can have either cationic or anionic terminations. It was found possible to determine the surface polarity of the HgCdTe(111) surface using XPD, thus suggesting a generally useful method for other epitaxial systems. To get a better understanding of XPD and the photoemission process, the directional dependence of energy-integrated valence-level spectra from W, which has strong direct transition effects, has been compared to analogous measurements on core levels. These data clearly show the importance of both direct-transition effects and the final-state angular momentum character of the photoelectron. The PD patterns for the usual elastica1ly-emitted core-level spectra were also compared with those of their associated plasmon losses to determine the effect of such inelastic scattering events on the PD patterns. Plasmon peaks were found to be strongly damped along low-index directions, and a theoretical model explaining this effect in terms of multiple scattering is proposed. Finally, the Si(111) surface was studied with photoelectron holography to help determine the utility of such FT techniques on these large scale data sets. Atomic images were observed for this surface, although there is an elongation of the atom-associated features along nearest-neighbor bond directions.